(C++）如何让它循环回到菜单?计算器类型

我知道这段代码仍然很草率，但我目前专注于让它在我让它变得漂亮之前做我需要它做的事情。我的任务是制作一个计算器，该计算器获取选择和输入，然后执行操作，然后循环回菜单以提示用户进行另一个选择和输入。我不完全确定如何在不使用全局变量的情况下做到这一点。

#include<iostream>
#include<iomanip>
using namespace std;
int showMenu(int & );
double getX(double & );
double getY(double & );
int add(double, double);
int divide(double, double);
int multiply(double, double);
int subtract(double, double);
bool exit(bool & );
int main(){
    int choice;
    bool yes = false;
    double x =0;
    double y =0;
    do{
        showMenu(choice);
        getX(x);
        getY(y);
        switch(choice){
            case 1:
                add(x,y);
                break;
            case 2:
                subtract(x,y);
                break;
            case 3:
                divide(x,y);
                break;
            case 4: 
                multiply(x,y);
                break;
            case 5:
                exit(yes);
        }
    }while(yes = false);
    return 0;
}
int showMenu(int &choice){
    cout << "*" << setw(50) << setfill('-') << "*" << endl;
    cout << "*" << setfill(' ') << right << setw(30) << "Calculator" << setw(19) << "*" << endl;
    cout << "*" << setfill ('=') << setw(50) << "*" << endl;
    cout << setfill(' ') << "* 1.) Add operation. " <<setw(30) << "*" << endl;
    cout << "* 2.) Subtract operation. " << setw(25) << "*" << endl;
    cout << "* 3.) Divide operation. " << setw(30) << "*" << endl;
    cout << "* 4.) Multiply operation. " << setw(30) << "*" << endl;
    cout << "* 5.) Exit. " << setw(40) << "*" << endl;
    cout << "*" << setfill('-') << setw(50) << "*" << endl;
    cout << "Please choose your option: " << endl;
    cin >> choice;
    if(choice < 0 || choice > 5){
        cin.clear(choice);
        cout << "Invalid selection." << endl;
    }
    return choice;
}
double getX(double &x)
{
    cout << "Input first number: " << endl;
    cin >> x;
    return x;
}
double getY(double &y)
{
    cout << "Input second number: " << endl;
    cin >> y;
    return y;
}
int add(double x, double y)
{
    double sum = x+y;
    cout << x << "+" << y << " = " << sum << endl;
    return 0;
}
int subtract(double x, double y)
{
    double mus = x-y;
    cout << x << "-" << y << " = " << mus << endl;
    return 0;
}
int multiply(double x, double y)
{
    double product = x*y;
    cout << x << "*" << y << " = " << product << endl;
    return 0;
}
int divide(double x, double y)
{
    double quotient = x/y;
    cout << x << "/" << y << " = " << quotient << endl;
    return 0;
}
bool exit(bool &yes)
{
    cout << "Goodbye!" << endl;
    yes = true;
    return yes;
}